688 MOLECULAR FORCES IN THE PLANT. 



exerted also against the inside of the vessels which receive the water from the turgid 

 parenchymatous cells. But the water which enters them has also to overcome the 

 resistance to filtration exercised by the cell-walls. The endosmose of the corkical cells 

 of the root must overcome these resistances. Although we do not know the magnitude 

 of the endosmotic force, yet we have ground for supposing that it is much greater than 

 that given by Dutrochet's experiments on animal membranes; and this explanation would 

 therefore be very probable. But a difficulty occurs in answering the question why the 

 turgescent cortical cells of the root expel their water only inwards into the woody tissues 

 and not also through their outer walls. We may however here be helped by the suppo- 

 sition that the micellar structure of the cell-walls is different on the outer and inner 

 sides of the cells, and that those facing the exterior of the root are best adapted to allow 

 endosmose, while those facing the interior of the root are best adapted for permitting 

 filtration under high endosmotic pressure. It must however be observed that this 

 supposition is at present only a hypothesis for the purpose of explaining to a certain 

 extent the processes which take place in the root. The exudation of drops of water 

 from the upper cell of the P^ungus Pilobolus crystallinus, from the root-hairs of a Mar- 

 chantla grown in damp air, &c., shows moreover that cells distended by endosmotic 

 tension can in fact exude water at certain spots. It is difficult to give any other ex- 

 planation of the exudation of nectar in flowers ; the excreting cells must evidently absorb 

 the water or the sap with great force on one side, and then exude it on the other side. 

 That in this case pressure from the root does not directly cooperate is shown by the 

 fact that this exudation of nectar, which is often very copious, as in the flowers of Fritil- 

 laria imperial is and in the pitchers of iV^/^'^'w/Z'^j^, takes place even when cut flowers or 

 pitchers are simply placed in water. In this respect these exudations of fluid differ from 

 the exudation of drops on the leaves of many plants, which only takes place when they are 

 still in connection with the root, and which is clearly caused by the forcing power of the 

 root (as in Aroideae, &c.). It also happens however sometimes that drops of water are 

 exuded from cut surfaces of the tissue, while another cut surface of the organ absorbs 

 water. This I found, for instance, to be the case with pieces of the young stems of 

 different Grasses, cut off from 6 to lo cm. in length, which were placed with the lower 

 end in damp soil ; the free upper end then repeatedly and continuously exuded drops of 

 water in darkness and in an atmosphere saturated with moisture. Here the paren- 

 chymatous cells of the lower cut surface clearly acted as the cortical cells of the root, 

 absorbed by endosmotic action, and probably pressed the water thus absorbed into the 

 vessels, from which it then escaped to the upper cut surface. 



(y) Tbe combined action of transpiration, conduction, and absorption of water by the 

 roots takes place under ordinary and favourable conditions in such a manner that nearly 

 as much water is absorbed through the roots and conducted upwards through the 

 wood as is transpired from the leaves. As long as this equilibrium lasts, the plant is 

 turgid and tense in all its parts ; and conversely, it may be concluded from the unaltered 

 turgidity and tenseness of the leaves and internodes that the conduction of water is 

 sufficient to compensate the transpiration by the leaves. Hence, under these conditions, 

 the quantity of water transpired may be taken as the measure of the absorption of the 

 root (or of a cut surface), and conversely the absorption observed as the measure of the 



^ Compare Wunschmann's dissertation, 'Ueber die Gattung Nepenthes' (Berlin, 1872), where my 

 Handbook only, and not the above taken from the third edition of this work, is quoted. It is 

 questionable if there is any ground for distinguishing between 'excretions' and the sap which escapes 

 from a root-stock in so far as the mechanism of the excretion is concerned, as the older botanists and 

 Wunschmann do. The facts above mentioned render it improbable. They tend to show, on the 

 contrary, that in other vegetable organs, as well as in roots, hydrostatic pressures may be set up 

 which tend to force the fluids out of the tissues. It is a matter of merely secondary importance that 

 'excretions,' such as nectar and the fluid contained in the pitchers oi Nepenthes, aie of higher concen- 

 tration than the sap which escapes from a root-stock. 



